Optical component and coating system for coating substrates for optical components

ABSTRACT

An optical component and a coating system for coating substrates for optical components with essentially rotationally symmetric coatings, the system having a planetary-drive system ( 1 ) that has a rotating planet carrier ( 2 ) and several planets ( 4 ), each of which carries a single substrate, that corotate both with the planet carrier and with respect to the primary carrier. In one embodiment a set of stationary first masks ( 20 ) that allow controlling the radial variation in physical film thickness is arranged between a source ( 8 ) of material situated beneath the planets and the substrates. A set of second masks that mask off evaporation angles exceeding a limiting evaporation or incidence angle (β max) for every substrate also corotate with the primary carrier ( 2 ), which allows depositing coatings having a prescribed radial film-thickness distribution and a virtually constant density of the coating material over their full radial extents for relatively low, and only slightly varying, evaporation angles.

This is a divisional application of U.S. application Ser. No. 10/244,419filed Sep. 17, 2002, now U.S. Pat. No. 6,863,398, issued Mar. 8, 2005,the disclosure of which is incorporated herein by reference. Thefollowing disclosure is based on German Patent Applications No. 101 46192.5 filed on Sep. 17, 2001 and No. 102 37 430.9 filed on Aug. 12,2002, which are incorporated into this application by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an optical component and coating system forcoating substrates for optical components with essentially rotationallysymmetric optical coatings and a coating system suitable for carryingout that method.

2. Description of the Related Art

Optical systems that have a plurality of lenses that must be coated withrotationally symmetric optical coatings in order to reduce theirreflectance or allow meeting other requirements are employed onprojection systems for the microlithographic fabrication ofsemiconductor devices and other types of microdevices. If necessary,other optical components, such as the imaging mirrors employed oncatadioptric or catoptric projection lenses, some of which may havesharply curved surfaces, will also have to be coated. These opticalcoatings should normally have an accurately controlled, usually asuniform as possible, optical effect over the entire optically utilizedportion of the coated surfaces involved.

The effects of coated surfaces in optical trains are largely dependentupon the film-thickness distributions of the coatings applied to thosesurfaces, i.e., in the case of rotationally symmetric coatings, theirradial film-thickness characteristic, which, in the following, will alsobe referred to as their “film-thickness distribution.” Flawedfilm-thickness distributions may have adverse effects, such as a falloffin the transmittances of lenses towards their perimeters, particularlyin the case of systems that have sharply curved optical surfaces. In thecase of field-lens systems, these effects may be worsened by effects dueto optical incidence angles. These incidence angles, which are alsotermed “incidence angles” or “i-angles,” differ at differing radiallocations on curved optical surfaces. While axial rays are normallyincident on the optical surfaces of lenses at their centers, some of therays striking their perimeters are incident at very large incidenceangles, where incidence angles ranging from 30° to 70° are fairly commonin the case of, e.g., projection lenses having high numerical apertures,which may lead to the optical properties of coated lenses being shiftedto shorter wavelengths compared to those at their centers. Variations inthe refractive indices of coating materials from the centers of coatingsout to their perimeters have already been observed, particularly in thecase of coatings on sharply curved optical surfaces, which also makesdesigning coatings difficult.

The aforementioned variation of the refractive index of coatings overtheir radii is largely due to a decline in the density of the coatingmaterial involved from their center out to their perimeter. Here theterm “density” refers particularly to the packing density, the opticaldensity and/or the mass density, which are related. The radial decreasein packing density causes other problems, particularly in the case ofsystems that operate with short-wavelength ultraviolet light. Forexample, porous coatings absorb more water than smooth coatings havingsuperior packing densities, which may lead to transmission problems at,for example, wavelengths less than about 280 nm, in particular, atwavelengths of 157 nm or less, since UV-light having a wavelength of 157nm is strongly absorbed by water, and coating-durability problems.Coating-adhesion problems near the perimeters of sharply curved, coated,optical surfaces have been observed.

The suspicion that has been voiced to date is that the observedvariations in the refractive indices of coating materials and supple andpoorly adhering coating structures near the perimeters of opticalsurfaces are attributable to the large incidence angles of coatingmaterial that occur near the perimeters of optical surfaces. Largeincidence angles have also been blamed for scattering losses and fordeleterious coating stresses at the perimeters of planar, evaporativelycoated, mirrors (cf. U.S. patent application Ser. No. A-5,518,518).

Coating systems equipped with planetary-drive systems are frequentlyemployed in order to allow, for example, simultaneously coating severalsubstrates in order to cut coating costs. A planetary-drive system ofthe type considered here has a primary carrier that may be rotated abouta primary rotation axis and is frequently referred to as a “planetcarrier” and numerous rotatable substrate carriers, each of which may berotated with respect to the primary carrier about a respectivesubstrate-carrier rotation axis, and are also termed “planets.” Whendepositing rotationally symmetric coatings, each substrate is clampedonto a substrate carrier such that the symmetry axis of the coatingsurface coincides with the primary rotation axis. Although the primaryrotation axis and the rotation axes of the substrate carriers areusually aligned parallel to one another, they may also be inclined withrespect to one another. The substrate carriers are arranged with respectto a material source, which is usually mounted on the primary rotationaxis, such that those locations on a surface of a substrate mounted on asubstrate carrier that face the material source and are to be coatedwill be coatable with coating material from the material source that isincident at incidence angles that may vary widely, particularly if thesurfaces to be coated are curved. Here the “incidence angles” or “anglesof incidence” for each location where a coating is to be deposited aredefined as the angles between the local normal to the coating surface atthat location and the direction of incidence of coating material at thatsame location, and usually vary with time.

Masking or baffling methods are employed in order to alleviate some ofthe aforementioned problems in the case of planetary-drive systems ofthis particular type. One example of a masking method is described inthe article entitled “Optical Coatings for UV Photolithography Systems”that appeared in SPIE Vol. 2775, pp. 335–365. Here masks inserted intothe planetary-drive system, between the material source and thesubstrates, that serve as shielding masks and have a special, computed,shape that allows intermittently masking off solid angles correspondingto high coating-deposition rates such that they yielded a desired,overall, film-thickness characteristic were employed. These correctionmasks are usually arranged in the vicinities of substrates. For everysurface shape and every desired film-thickness characteristic, there isan optimal mask geometry that may be computed.

Since it has been found that conventional masking methods are not alwaysthe ideal choice, particularly in cases involving coating sharply curvedoptical surfaces intended for use in applications involving wavelengthsfalling within the deep-ultraviolet spectral range and shorterwavelengths, there is need for further improving these sorts of maskingmethods.

SUMMARY OF THE INVENTION

It is one object of the invention to provide a coating method that willallow depositing coatings which act uniformly over the entire coatedsurface, particularly on sharply curved substrates intended for use inapplications involving the short-wavelength ultraviolet spectral region.It is a further object to provide a coating system suitable for carryingout that method.

As a solution to these and other objects, the invention, according toone formulation, provides a method for coating substrates for opticalcomponents with essentially radially symmetric optical coatings, themethod including:

-   -   arranging at least one substrate with respect to a material        source such that coating locations on a coating surface of the        substrate facing the material source will be coatable by coating        material emanating from the material source;    -   rotating the substrate about a substrate rotation axis;    -   controlling a radial film-thickness characteristic of the        coating such that quantities of material incident on coating        locations on the coating surface are adjusted depending on the        radial distances of the coating locations from the substrate        rotation axis:    -   defining a limiting incidence angle that represents a maximum        tolerable incidence angle; and    -   controlling the incidence angle by actively limiting the        incidence angle such that the incidence angle of coating        material at every coating location on that coating surface does        not exceed the limiting incidence angle at any time during        coating.

Beneficial embodiments are as stated in the dependent claims. Thewording of all claims appearing herein is herewith made an integral partof this description by reference.

In the method according to the invention, at least one substrate isarranged with respect to material source arranged within the coatingsystem such that coating locations on a coating surface (i.e. a surfaceto be coated), that are to be coated facing the material source will becoated with coating material emanating from the material source that isincident thereon at incidence angles that will normally vary with timeat a given coating location. The substrates are rotated about asubstrate rotation axis. That rotation is essential to the rotationalsymmetry of the resultant optical coating and provides that the symmetryaxis of the rotationally symmetric coating will coincide with thesubstrate rotation axis. Although a symmetry axis of the opticalcomponent involved, if any, will also normally coincide with thesubstrate's rotation axis, it may also be inclined at an angle theretoor be displaced parallel thereto. Control over a radial film-thicknesscharacteristic of the optical coating involved is exercised such thatthe quantity of coating material striking the coating surface will varywith radial distance of the location being coated from the substrate'srotation axis. Control over local incidence angles of coating materialis also exercised in order that they will not exceed a prescribedlimiting incidence angle at any location to be coated on the coatingsurface. Coating is thus actively confined to a relatively narrow rangeof incidence angles, where this active confinement provides that themaximum incidence angle that actually occurs will be less than themaximum possible incidence angle, as determined by substrate geometry,system geometry, and the type of system design involved, that wouldoccur without this active confinement.

This confinement of coating to low incidence angles, in particular, tomore or less normal incidence or acute incidence angles, of coatingmaterial on the surfaces of substrates provides that the resultantcoatings will have uniformly high packing densities and low porositiesin order that absorption of water and other contaminants will be largelyprecluded, or, in any event, at least reduced. Variations in therefractive index of the coating material will also be avoided, and therefractive index of the coating material will more nearly equal therefractive index of the bulk material involved than in the case ofconventionally deposited coatings, which will greatly simplify coatingdesign. Coating-adhesion problems at the perimeters of coated opticalcomponents will also be avoided, which will extend their service lives.

A combination of accurately controlling coating film-thicknessdistribution and avoiding large coating-material incidence angles isthus proposed. This combination usually requires modifying the measures,i.e., for example, the shapes, sizes, and numbers of shielding masks,for controlling radial film-thickness characteristics employed underconventional baffling methods, since, in determining the quantities ofmaterial that need to be evaporated in order to yield a desired radialfilm-thickness characteristic, due account must be taken of the factthat only materials that is incident at relatively small incidenceangles will contribute to coating deposition. For similar types ofsubstrates, coating processes in accordance with the method inaccordance with the invention thus usually require using more coatingmaterial in the source than conventional methods. However, this minordisadvantage is more than offset by major benefits in terms of theoptical and physical properties of the resultant coatings.

The method according to the invention is suitable for depositing nearlyrotationally symmetric optical coatings on the surfaces of substratesfor optical components, such as lenses or mirrors, that are to becoated, which is preferably carried out in a coating system equippedwith a planetary-drive system for moving the substrates during coating.Although the method is specifically intended and suitable for coatingsubstrates whose surfaces to be coated are sharply curved, it may alsobe employed for coating substrates having nearly planar surfaces orsurfaces that are only slightly curved.

Another embodiment of the method provides that, in controlling theirradial film-thickness distribution, sections of surfaces to be coatedare intermittently shielded from the material source, where a durationof the shielding intervals involved as a function of the radial distanceof a location to be coated from the substrate's rotation axis and,preferably, also as a function of the maximum tolerated incidence angle,is set. A first control device having at least one mask that may bearranged between the material source and the substrate and whose shapehas been chosen, in particular, though suitable computations, such thatshielding intervals of the required duration will result, may bearranged in the coating system for that purpose. Preferred are numerous,permanently mounted, stationary, first masks that may be arranged aroundthe circumference of a common circle encircling the primary rotationaxis. Providing a large number of masks, where the individual maskspreferably all have the same shape, will be best if the source islocated on the primary rotation axis, which will simplify computing theshapes of, and fabricating, the masks and stabilize the initial andterminal conditions of each individual planet. A suitable choice oflocation for the source, e.g., on the primary rotation axis or slightlyoffset with respect thereto, will render the coating process insensitiveto processing errors, such as off-axis evaporation of the coatingmaterial.

A shielding of critical sections of surfaces to be coated from thematerial source is preferably employed for controlling incidence angles,where “critical sections” are defined as sections thereon wherehypothetical incidence angles resulting from the geometry of thesurfaces to be coated and their spatial arrangements with respect to thematerial source exceed the prescribed limiting incidence angle.

In the case of a coating system equipped with a planetary-drive system,some embodiments provide, for each substrate carrier, a second mask thatmay be arranged between the material source and the substrate, whoseshape is such that it yields this shielding against excessively largeincidence angles. These second masks may also all have the same shape.In cases where convex surfaces are to be coated, these preferably havean inner edge facing the primary rotation axis that curves radiallyoutward. The overall shape of these masks may resemble a crescent,particularly if the material source is arranged on the primary rotationaxis. In cases where concave surfaces are to be coated, their inner edgepreferably curves outward, away from the primary rotation axis.

In the case of other embodiments, the second control device, whichserves to preclude excessively large incidence angles, a shielding maskhaving a rim that is eccentrically disposed with respect to the materialsource over one or more sections of its circumference and encircles theprimary rotation axis is arranged between the material source and thesubstrates. The rim of this mask is thus not quite concentric with thematerial source. It has been found that minor departures from totallyrotationally symmetric coating conditions may be beneficial, in thatthey allow creating coating systems that are relatively insensitive tominor misalignments of the spatial arrangements of their material sourceand shielding masks.

The rim or edge of this mask should be tangent to, or nearly tangent to,straight lines passing through the vertices of the substrates and thepoint where the primary rotation axis intersects the plane of thematerial source. It will be beneficial if this shielding mask isarranged not too close to the substrates and at a sufficiently largedistance from the material source. Mask heights that range from 20% to50% of the (vertical) distance between the material source and thesurfaces to be coated are particularly beneficial.

A slight decentering of this shielding mask may be achieved by, forexample, providing that it has a circular rim centered on a (vertical)axis and that that axis is arranged such that it is off-axis withrespect to an axis passing through the material source, parallel to theprimary rotation axis, by an eccentricity offset. This eccentricity ofthe material source with respect to the rim of the mask may be achievedin various ways. For example, the mask, or the rim of the mask, may bearranged concentric with the primary rotation axis and positioned suchthat it is off-axis with respect to the material source, i.e.,positioned such that it is laterally displaced from the point where theprimary rotation axis intersects the plane of the material source. Themask may also be positioned such that it is off-axis with respect to theprimary rotation axis and concentric with the material source. Employinga combination of the two is also possible. Instead of a flat mask thatmay, for example, be arranged such that it is nearly horizontal,cylindrical masks may also be employed. In the case of such latterarrangements, this slight eccentricity will be beneficial, in that itallows avoiding a singular behavior of the film thickness at the centerof the normally spherical, or aspherical, curved surfaces to be coatedthat, in the case of other types of system, may result from slightmisalignments of their masks.

Employing an eccentricity offset that is small compared to the radialdistance between the primary rotation axis and the rotation axes of thesubstrate carriers has been found to be beneficial. Such slightlyoff-axis positionings of the material source may, for example, occurwhen this eccentricity offset is less than 30%, in particular, less than20%, of that radial distance. It will usually be beneficial if thiseccentricity offset is more than 1% or 2% of that radial distance.

The shielding mask may also be configured such that it has a slightlyelliptical rim or edge. A mask of that type may, for example, bearranged such that it is concentric with respect to the primary rotationaxis and employed in conjunction with a material source that isconcentric with respect to the primary rotation axis. The ellipticalshape of the rims of masks of this sort also yields slight departuresfrom totally rotationally symmetric coating conditions, since their rimsare nonrotationally symmetric, and therefore, in that sense, eccentric.

Another means for avoiding excessively large particle incidence anglesmay be implemented by providing the coating system with a device forrotating the shielding mask about a vertical rotation axis, which will,for example, allow employing an off-axis shielding mask, in particular,a shielding mask having a circular rim, and rotating it about thecoating chamber's primary rotation axis, i.e., rotating it relative tothe material source. The material source may be arranged such that it isconcentric with the coating chamber's primary rotation axis. Rotatingthis annular mask will allow compensating for the lack of rotationalsymmetry in coating conditions due to its off-center rotation axis. Arotationally symmetric masking effect due to employment of this mask,may be obtained on a time-averaged basis, without running the risk ofincurring singularities in the film thicknesses at the centers of thesurfaces to be coated.

Alternatively, or in addition, thereto, a device for tilting therotation axes of substrate carriers may be provided. Combining an, forexample, annular, shielding mask with an additional tilting of thesubstrate carriers (planets) will allow minimizing variations inparticle incidence angles over the surfaces to be coated.

Simultaneously coating substrates employing several off-axis sources ofmaterial that may be positioned, e.g., around the circumference of acircle encircling the primary rotation axis, and employing a concentric,annular, shielding mask having circular inner and outer rims may alsoallow achieving approximately rotationally symmetric coating conditionsin the vicinities of substrates.

A material source that rotates about the coating system's primaryrotation axis, which will allow obtaining similar, beneficial, effectson the uniformity of coating conditions as the aforementionedembodiment, may also be provided.

In the case of those embodiments described here, which have masks withcircular rims, the shielding mask may be annular and have a circular, orslightly elliptical, inner rim. Such embodiments are particularly suitedto coating substrates having convex surfaces. In cases where concavesurfaces are to be coated, the shielding mask should be configured suchthat its outer rim does the shielding. The shielding mask may be formedfrom, for example, a nearly circular, or slightly elliptical, piece ofsheet metal that may be installed concentric with, or slightly off-axiswith respect to, the primary rotation axis.

In order to achieve dense, strongly adhering, coatings, the incidenceangles of streams of material emanating from the material source shouldbe as small as possible and undergo the slightest possible variations atgiven locations on surfaces to be coated. In the case of a preferredembodiment of the method, a limiting incidence angle that is at least10%, and preferably at least 20%, less than the maximum-possibleincidence angle, based on the coating geometry involved, is set. Highlypreferred are incidence angles that do not exceed 60° or 45°. Therespective optimal mean incidence angles are highly dependent upon thecoating materials involved and the operating wavelengths of the coatingsinvolved. The minimum mean incidence angle that may be attained varieswith coating-system geometry. It will be particularly beneficial ifcoating takes place at a mean incidence angle, which should preferablybe minimized, that remains virtually constant over all locations to becoated.

In that conjunction, it has been found that it may be beneficial if thelimiting incidence angle set is based on the type of coating materialinvolved.

The invention allows creating coatings whose densities fall within therange ρ₀±20%, in particular, fall within the range ρ₀±10%, where ρ₀ isthe mean material density of the coating material involved at the centerof the coated surface or at the location where the minimum meanincidence angle occurs. Refractive-index variations and risks thatcoatings will absorb water or other contaminants may be held withintolerable bounds under these conditions.

One major benefit provided by the invention is that avoiding excessivelylarge evaporation angels allows avoiding the associated coating errors,such as refractive-index variations, while simultaneously allowingaccurately controlling the radial film-thickness distributions ofcoatings over broad ranges. In the case of coatings deposited on curvedsurfaces, their film thicknesses would monotonically decrease from theircenters out to their perimeters by amounts that would vary with theircurvatures in the absence of control measures. Conventional means ofcontrol, such as employing conventional masking methods, have frequentlybeen utilized for compensating for this decline in film thickness inorder to allow arriving at nearly uniform physical film thicknesses overthe entire radial extents of coatings. However, that approach may leadto the optical properties of coatings varying widely from their centersout to their perimeters, in spite of their uniform physical filmthicknesses, particularly in the case of systems that involve largeincidence angles on their optical components. For example, thereflectance minima of anti-reflection coatings will be shifted toshorter wavelengths in such cases. In the case of a preferred embodimentof the method, a radial film-thickness characteristic, whereby thephysical film thickness of a coating monotonically increases from thelocation of its symmetry axis out to its perimeter, is created, whichallows providing that the optical effect of the coating will bevirtually independent of the distribution of incidence angles involved.Its thickness, which varies with radial location, may, for example, bethe result of multiplying its thickness at its center by a correctionfactor that may, for example, follow an approximately parabolic curvewhose vertex is situated at its center. For example, its thickness atits perimeter might exceed that at its center by 5% to 10% or more.

The invention also relates to a coating system suitable for carrying outthe method in accordance with the invention that has a planetary-drivesystem for moving substrates during coating, where its planetary-drivesystem has a primary carrier (planet carrier) that may be rotated abouta primary rotation axis and numerous substrate carriers (planets), eachof which may be rotated with respect to the primary carrier about arespective substrate-carrier rotation axis. The coating system comprisesa first control device for controlling the radial film-thicknesscharacteristic and a second control device for limiting incidence anglesto a maximum tolerable incidence angle. There are embodiments whosefirst control device has a number of first masks that may be arrangedbetween the material source and the substrates and whose second controldevice has a number of second masks that corotate with the primarycarrier in order to mask off large incidence angles. Embodiments whosesecond control device is configured such that small departures fromtotally rotationally symmetric coating conditions may be achieved arealso feasible. Examples of such embodiments include systems havingcentrally mounted, annular or circular, shielding masks, combined with aslightly off-axis material source or systems having either annular orcircular shielding masks mounted slightly off-axis combined with acentral material source or centrally mounted or slightly ellipticalshielding masks combined with a central or slightly off-axis materialsource, as well as combinations thereof. Embodiments having rotatingshielding masks and/or tiltable substrate carriers are also utilizablefor the same purposes.

The foregoing and other characteristics of the invention are as statedin both the claims and the description and depicted in the accompanyingfigures, where the individual characteristics involved may representcharacteristics that are patentable alone or several such in the form ofcombinations of subsets thereof that appear in an embodiment of theinvention and may be implemented in other fields, as well as beneficialembodiments that may themselves be patentable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematized representation of a first embodiment of aplanetary-drive system in accordance with the invention employed fordiscussing coating geometry;

FIG. 2 is an angular-perspective top view of an embodiment of aplanetary-drive system in accordance with the invention having first andsecond masks for affecting coating properties;

FIG. 3 is a schematic diagram showing various radial variations in thephysical thickness of rotationally symmetric coatings;

FIG. 4 is a schematic diagram showing various radial variations in thedensity and refractive index of rotationally symmetric coatings;

FIG. 5 is a schematized section through a lens having a sharply curvedsurface onto which a coating deposited in accordance with the inventionhas been deposited, together with a schematic plot of the radialvariations in the physical thickness and mass density of that coating;

FIG. 6 is a schematized representation of a second embodiment of aplanetary-drive system according to the invention having an annularshielding mask and an off-axis material source;

FIG. 7 is a diagram showing computed mean values of particle incidenceangles as a function of radial location on a coating surface for acoating deposited without using a shielding mask and a coating depositedusing an on-axis, annular, shielding mask for various eccentricityoffsets of the material source;

FIG. 8 is a diagram of relative coating thickness as a function ofradial distance from its center for the same set of coating conditionsstated in conjunction with FIG. 7;

FIG. 9 is a diagram showing the effects of tilting the axes of planetson variations in local mean particle incidence angles over the radialextent of a coating surface; and

FIG. 10 is a schematized representation of an embodiment of aplanetary-drive system for use in coating concave surfaces.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 schematically depicts the geometric arrangement of majorcomponents of an evaporation system with which optical components formicrolithographic projection systems, in particular, lenses havingsharply curved surfaces, may be coated using electron-beam evaporationmethods or other PVD-methods, including sputtering, where the extensionof the material source is preferably small compared to the coatingsystem's dimensions, i.e. where the material source represents aquasi-point source. Since it may be desirable to simultaneously coatseveral substrates, which will normally be of similar type, underlargely identical processing conditions on costing or technologicalgrounds, the coating system has a planetary-drive system 1 situatedwithin an evacuable coating chamber that is not shown here for movingthe substrates to be coated during coating. This planetary-drive systemhas a nearly circular, disk-shaped, primary carrier 2 that is alsotermed a “planet carrier” and may be rotated about a vertical primaryrotation axis 3 by a drive that is not shown here. This planet carrierhas five, or some other number of, identically configured substratecarriers 4 uniformly distributed around its circumference, each of whichis mounted such that it may be rotated about a verticalsubstrate-carrier axis 5. The primary carrier 2 is mounted above a driveshaft gear that is not shown here and is intermeshed with planet gearsattached to the planets such that the rotation rate of the planets is afunction of the rotation rate of the driven primary carrier and the gearratio of the drive shaft gear and planet gears. A substrate carrier willthus usually execute several rotations about its axis 5, which rotatesabout the primary carrier's rotation axis as the substrate carrierrotates, during a single rotation of the primary carrier. A source 8 ofmaterial containing the coating material that, in the case of theexample shown, is evaporated by an electron beam, is located beneath theplanets, at a suitable distance therefrom.

In the case of the deposition of rotationally symmetric coatings to bedescribed here, every planet has a substrate 10 to be coated, which, inthe case of the example shown here, is a biconvex lens having a diameterranging from about 10 cm to 30 cm, fastened to its underside. Thesurface 11 of the lens that is to be coated is sharply curved. The ratioof its diameter, D, to the radius of curvature, R, of the coatingsurface may, for example, be less than −2/3 or more than +2/3, where theoptimal ratios for concave and convex surfaces will usually differ. Inparticular, the optimal ratio, D/R, for concave surfaces might be givenby |D/R|>1.3, and preferably by |D/R|>1.6, while that for convexsurfaces might be given by |D/R|>0.3, and preferably by |D/R|>0.67. Thelens 10 is fastened to a substrate carrier 4 such that the center ofsymmetry Z of the coating to be deposited lies on the symmetry axis ofthe substrate carrier 5, which, in the case of the example shown here,also coincides with the symmetry axis of the substrate. The point P₀designates the center of the coating surface and the point P₁ lies onits perimeter. The parameter φ parameterizes the surface 11 andincreases with increasing radial distance of a point P from the centerof symmetry Z of the coating to be deposited.

If the coating material is heated at the location of the source 8 ofmaterial until it evaporates, the material source will emit a beam ofevaporated material that will be primarily directed upward and strikethe substrates 10 fastened to the planets and other objects situated inits path, thereby yielding specified rates of deposition of the coatingmaterial for every location P to be coated on the coating surface thatwill vary with its positioning relative to the material source. In thecase of the sample situation shown in FIG. 1, the coating depositionrate, CDR, i.e., the quantity of coating material deposited per unittime, at the location P will be given by the following equation:

${{{CDR}(P)} = {K \cdot \left\{ \frac{{\left\lbrack {{{\left( {1 - A} \right) \cdot \cos^{n}}{\alpha(P)}} + A} \right\rbrack \cdot \cos^{m}}{\beta(P)}}{{r(P)}^{2}} \right\}}},$where K is a constant that depends upon the type of material sourceinvolved, A and n specify properties of the material source, α is theangle between the beam 13 of vapor (indicated by the dotted lines)emanating from the source 8 of material and directed at the point P andthe symmetry axis of that source of evaporated material, which will, inthe ideal case, coincide with the primary rotation axis 3. The parameterβ specifies the incidence angle of material coming from the materialsource at the point P, and is defined as the angle between the localnormal 14 to the surface at that point and the direction 13 of the beamof vapor at point P. The parameter m is an exponent specifying thecondensation characteristics of the coating material for givenprocessing parameters, such as substrate temperature, and is usually setequal to 1. However, the exponent m may also be less than 1 for somesubstrate materials and coating materials. For example, m might rangefrom about 0.5 to 1. The parameter r specifies the distance between thesource 8 of material and the coating location P.

From FIG. 1, it may be seen that the incidence angle β for every point Plying outside the substrate rotation axis 5 will vary with time as thesubstrate is rotated. In the case of the curved surface 11 to be coatedshown here, which is convex and faces downward, this incidence angle βwill be relatively large whenever P lies outside, referred to the source8 of material, the rotation axis 5 and relatively small whenever thepoint P lies inside that axis 5, i.e., on the side of that axis thatfaces toward the source 8 of material, as will occur, e.g., when thesubstrate shown is rotated through 180°. The opposite will apply if thecoating surface is concave, as in the case of, for example, concavemirrors.

It may also be seen that incidence angles at locations on, or near, therotation axis 5 of the substrate remain virtually constant when thesubstrate is rotated, while variations in incidence angle increase withincreasing radial distance of the coating location from the symmetryaxis, where the magnitudes of incidence angles, and the extents to whichthey vary, are largely dependent upon the curvature of the coatingsurface and increase with increasing curvature.

“Self-screening” of surfaces to be coated may also occur, particularlyif the surfaces to be coated are sharply curved, in which case, sections15 lying beyond the intersections of straight lines 16 tangent to thesurfaces to be coated with those surfaces at a given point in time willnot be directly accessible to the beam of evaporated material emanatingfrom the source 8 of material. These tangent lines 16 pass through thosepoints on substrates where the beam of evaporated material emanatingfrom the source 8 of material just grazes their surface (β=90°) for agiven set of processing parameters.

Whenever curved surfaces are coated using such a planetary-drive system,it is observed that the physical thickness d of the resultant coatingdecreases from the its centroid (center of symmetry Z) out to theperimeter of the substrate, where this effect becomes more pronouncedfor more sharply curved surfaces. How prominent this effect is maydiffer for concave and convex surfaces. This effect is schematicallyindicated by the dotted curve 17 in FIG. 3, which presents plots of theratio of the thickness d of the coating to its thickness do at itscenter of symmetry as a function of radial distance Rad from its centerof symmetry. This radial decrease in coating thickness adversely affectsthe optical properties of such coatings in the sense that their opticalproperties near their perimeters are frequently worse than at theircenters, the location for which the coatings have usually been designedor computed.

In the case of the conventional masking or screening or masking method,influence of these coating flaws is reduced by employing first masks 20that serve as shielding masks, between the source 8 of material and thesubstrates, and are usually arranged in the vicinities of thesubstrates, i.e., immediately below those surfaces to be coated, on,e.g., a stationary, annular, mask holder 21. In the case of theembodiment shown in FIG. 2, six masks of that type that all have thesame, identical, shape and are arranged around the circumference of theplanetary-drive system are provided. The shape of these masks, which, inthe case of the example shown, is a teardrop shape, is computed suchthat they will intermittently interrupt the stream of material flowingfrom the source 8 of material to the surfaces of the substrates when thesubstrates rotate about their axes and about the primary rotation axis3, where the temporal durations of the resultant interruption intervalsare normally chosen such that they are longer for radial segments wheredeposition rates are extremely high than for radial segments wheredeposition rates are lower. In particular, employing this approach willallow subjecting segments near the perimeters of substrates to thestream of material for longer periods than segments lying furtherinward, where the mass flow rate of the stream of material is greater,which will allow controlling the radial film-thickness distribution. Thesolid curve in FIG. 3 schematically indicates the normalized radialfilm-thickness characteristic for a curved surface when these stationaryfirst masks 20 are employed, where the shape of these masks has beenchosen such that the thickness d of the resultant coating remainsessentially constant over the full radial extents of coated surfaces.

Employing these stationary first masks 20 will thus allow affecting thecourse of the physical thickness d of the coating from its center out toits perimeter. However, coatings of that type, nevertheless, frequentlyfail to have the desired optical properties and exhibit intolerabledifferences in their properties between their center and theirperimeter. In particular, variations in their densities and theassociated variations in their refractive index, i.e., that coatings aremore dense and more firmly compacted at their centers and that surfacecoverage, or their packing density, decreases out to their perimeter,are observed. Adhesion problems may also occur near their perimeter. Inorder to clarify these problems , FIG. 4 presents plots of the massdensity, ρ, of a coating, normalized to the mass density, ρ₀, at itscenter of symmetry, as a function of radial distance Rad from its centerof symmetry. The dotted curve 23 schematically indicates the decrease inmass density from the center to the perimeter of a coating depositedeither without employing shielding masks or a coating depositedemploying the stationary masks 20 described above. The refractive indexn of such a coating, normalized to its refractive index, n₀, at itscenter, exhibits a similar behavior, since, to a first approximation,refractive index is proportional to mass density. The relationshipbetween refractive index and mass density is given by, e.g., theLorentz-Lorenz equation, which is well-known to experts in the field.

These problems may be avoided by employing the invention, where, inaddition to stationary masks 20, the planetary-drive system shown inFIGS. 1 and 2 has a number of second masks 25, whose number equals thenumber of planets employed, mounted on the primary carrier 2 andinclined at a suitable angle 26 with respect thereto that corotate aboutthe primary rotation axis 3 with the primary carrier, for that purpose.These second masks are mounted a short distance above the first masks20, between the source 8 of material and the substrate that is fastenedto each planet. These crescent-shaped second masks 25 all have the same,identical, shape and an arced inner edge 27 facing the primary rotationaxis 3 that curves away from the primary rotation axis 3.

The shape of the inner edge 27 of each of these masks is defined by thelocus of those points on the second mask plane 28 containing the secondmasks 25 where that plane is intersected by straight lines passingthrough the source 8 of material and those points P on the surfaces ofthe substrates that correspond to the maximum incidence angle, orlimiting incidence angle β_(max), specifically chosen for the particularcase involved, at a given point in time. In particular, it may be seenfrom FIG. 1 that the inner rims 27 of these masks will be shifted closerto the primary rotation axis 3 as the desired limiting incidence angleβ_(max) is decreased. Since experts in the field are able to accuratelycompute the shapes of the inner rims of these masks based on geometricconsiderations, their computation will not be further discussed here.

These crescent-shaped masks represent merely one embodiment among manypossible embodiments that also meet the condition regarding limitingmaximum incidence angles in order to limit mean incidence angles. Inorder to limit mean incidence angles, it will be sufficient to boundthese masks by an imaginary, averaged, curved line in a manner such thatundershootings and overshootings of the shielding of incidence anglesdue to these masks, or a function thereof, e.g., cos^(m) (incidenceangle), will average out in the arithmetic sense.

In addition to the aforementioned “simple crescent” shape, masks havingthis same sort of shape may also be configured such that the contour oftheir inner edge is defined by a pair of axisymmetric arcs. Otherembodiments that meet the aforementioned condition may be computed. Theadvantage of employing masks that have shapes that differ from thissimple crescent shape is that they eliminate “kinks” in coatings'd/d₀-curves, where d₀ is their thickness at the centroid of thesubstrate. Correcting for such “kinks” by employing stationary masksnecessitates employing masks having jagged edges and thus imposes highpositioning accuracies on the arrangement.

Experts in the field will recognize that this shielding of incidenceangles that exceed the limiting incidence angle β_(max) reduces thetotal quantity of material deposited per unit time, which may becompensated for by suitably contouring the first masks 20, which may,for example, have narrower lateral widths at radially outlying locationsthan comparable conventional masks in the absence of any corotatingmasks 25 of the second type. The resultant shortened shielding intervalswill then compensate for the shielding effect of the second,crescent-shaped, masks 25.

An effort should be made to choose the geometry of the second mask 25such that mean incidence angles will remain virtually constant overthose surfaces to be coated. Mean incidence angles should preferably beminimized, which implies that a stream of material that is normallyincident on surfaces to be coated is preferred. In the case of preferredmethods, the minimum incidence angle on surfaces to be coated that maybe achieved occurs at their centroids, and may range from about 20° toabout 25°.

The opportunities for avoiding large incidence angles afforded by theinvention allow depositing evaporated coatings for which the massdensity of the deposited material remains approximately constant fromtheir center of symmetry out to their perimeter and varies by no morethan, for example, 10% to 20%, from their center of symmetry to theirperimeter. In FIG. 4, the solid curve schematically indicates onepossible radial mass-density distribution resulting from limitingincidence angles that, in principle, corresponds to the coating'srefractive-index distribution.

Another beneficial embodiment of the invention will now be discussed,based on FIG. 5. FIG. 5 depicts a sectional view of a portion of a lens10 fabricated from, for example, synthetic quartz glass or a fluoridecrystal, whose sharply curved surface 11 has an optical coating 30 thatis rotationally symmetric with respect to its axis of symmetry Z., whichis also the axis of symmetry of the circular lens 10. In evaporatingthat coating 30, which might consist of, e.g., one or more dielectricand/or metallic layers, the shape of the first masks 20 was chosen suchthat, on a time-averaged basis, more material was deposited in thevicinity of the perimeter of the lens than in the vicinity of its axisof symmetry, which yields a coating having a varying radial thickness dthat monotonically increases from its center of symmetry out to itsperimeter, where its thickness at its perimeter might, for example, be10% greater than its thickness d₀ at its center.

Simultaneously, employment of the corotating second masks 25 providedthat material deposited on the lens' surface was incident thereon atsmall mean incidence angles over its entire radial extent, where maximumincidence angles were limited to, for example, 40°, which yielded acoating 30 having a practically uniform mass-density distribution fromits center (mass density d0) out to its perimeter, so that therefractive index of the coating material is also virtually independentof radial location on its coated surface (cf. the plots thereofappearing in FIG. 5).

In the case of the example shown, where the thickness of the coatingincreases out to its perimeter, the radial variation of its thicknesshas been chosen such that the shift (phase shift) in coating propertiestoward shorter wavelengths at the perimeter of the lens due to thelarger incidence angles occurring there that is observed for uniformlythick coatings is accurately compensated. The optical properties of thecoating, in particular, the location of the reflectance minimum causedby the coating, are thus practically independent of radial location onthe lens and the local incidence angles that occur there. In otherwords, the phase shift for the various rays penetrating the coating 30remains virtually constant along the radial direction. The thicknessgradient required to achieve that constancy depends upon the curvatureof the surface 11 to be coated, the refractive index of the coatingmaterial, and the incidence angle (i-angle) of the incident radiation.Since, thanks to the invention, the refractive index of the coatingmaterial may be made virtually independent of location, coating designand coating deposition are greatly simplified, and better-qualitycoatings may be deposited.

Another embodiment of a planetary-drive system 101 that has a shieldingmask 127 for reducing mean incidence angles, will now be presented,based on the schematic representation appearing in FIG. 6. Itsdimensions, which are intended to represent an example only, are suchthat it is suitable for use on systems for coating large-diameter lensesand similar having diameters ranging up to, for example, 30 mm. Thissystem has a set of first shielding masks that are not shown in thefigure for controlling the radial film-thickness distributions ofdeposited coatings. The shape and positionings of these masks maycorrespond to those of the masks 20 of the first embodiment. The simplydesigned second control device for actively limiting particle incidenceangles has an annular shielding mask 127 that is installed in thecoating system such that lies in a horizontal plane. The inner rim 130of this mask 127 circumscribes a circle about the primary rotation axis103, which is coincident with the axis of the mask's rim, and is thuscentered with respect to the primary rotation axis. This stationaryshielding mask 127 is installed roughly halfway between the plane of thesource 108 of material and the vertices P₀ of the spherical surfaces 111to be coated such that a straight line passing through the point wherethe primary rotation axis 103 intersects the plane of the source 108 ofmaterial and those vertices just barely grazes the inner rim 130 of themask 127, which provides that coating of all areas on those sphericalsurfaces 111 that face away from the primary rotation axis 103, i.e.,areas where the largest particle incidence angles would occur, will bemasked off at all times.

In the case of this sort of mask geometry, centering the source 108 ofmaterial on the primary rotation axis 103 might give rise to the problemthat the film thickness at the centers P₀ of those surfaces to be coatedwould be either zero or approximately double that occurring atimmediately adjacent areas, depending upon whether the centers of thesurfaces to be coated were, or were not, respectively, being momentarilyshielded by the masks. Even a slight maladjustment of the height and/orcentering of the mask 127 would be sufficient to cause one or the otherlimiting case to occur and lead to a singularity in the film-thicknessdistribution occurring near the centers of substrates.

These problems are avoided by introducing a controlled, slight,departure from totally rotationally symmetric coating conditions. In thecase of the example shown, that introduction is achieved by arrangingthe source 108 of material such that it is offset from the primaryrotation axis 103 by an eccentricity offset 131. This slightly off-axislocation of the material source eliminates risks of the aforementionedsingular behavior of the film thickness at the centers of substratesoccurring. Providing a moderate separation of, for example, 50 mm,between the material source and the primary rotation axis here will beboth sufficient and beneficial. In general, this eccentricity offset 131should range from around 1% –2% to around 20% or 30% of the radialdistance 132 between the primary rotation axis 103 and the rotation axes105 of the substrate carriers.

The mode of operation of the annular mask 127 for slightly off-axislocations of the evaporator is indicated by the dotted line in FIG. 6.The centers P₀ of some of the surfaces to be coated, as well as allother points thereon, will be coated, while the centers of others, aswell all other points thereon, will be shielded. Although nosingularities in the film-thickness distributions at the centers P₀ ofsubstrates will occur during coating, points P₁ in the vicinity of theperimeters of the convex surfaces 111 to be coated will be shieldedwhenever the largest particle incidence angles on their sides that faceaway from the primary rotation axis would occur.

The effectiveness of this annular shielding mask 127 when employed inthe geometric arrangement shown will now be demonstrated, based on FIG.7. FIG. 7 presents plots of the mean particle incidence angles β thatoccur when coating concave, spherical, surfaces using a planetary-drivesystem 101 for the case where no masks are employed (curves a and b) andthe case where a centered, annular, mask 127 is employed (curves c, d,and e) for various positions of the source 108 of material as a functionof radial distance, or angular displacement φ, from the center of thesubstrate involved. In computing these mean particle incidence angles,the local mean particle incidence angles at various points P on thespherical surface 111 of the substrate while the planetary drive was inmotion were computed at time intervals of 0.01 second and then averaged.It may be seen that the position of the material source in the case ofcoating involving no masks (curve a, which has been computed for thecase of a centered evaporator, and curve b, which has been computed forthe case of an eccentricity offset of 100 mm) has only an extremelyslight effect on mean particle incidence angles, which increase along aroughly parabolic curve with increasing radial distance, or angulardisplacement φ, from the center of the substrate. A comparison of areasnear the perimeters of coatings deposited on spherical surfaces for thecase where the material source was shifted off-axis by as much as 50 mmand an annular mask was employed and the case where no annular mask wasemployed clearly indicates the effectiveness of the annular mask inreducing mean incidence angles. The curves c, d, and e correspond toeccentricity offsets of 1 mm, 50 mm, and 100 mm, respectively. Forφ>20°, mean particle incidence angles may be reduced by about 12° byemploying the shielding mask 127. If the eccentricity offset is furtherincreased to, for example, 100 mm (curve e), the effectiveness of theannular mask is reduced. The effectiveness of this mask may thus beoptimized by choosing a suitable eccentricity offset.

The effect of the shielding mask on coating thickness d, expressed inrelative units, for the case of those coating conditions stated above inconjunction with the discussion of FIG. 7 will now be discussed, basedon FIG. 8. In the case of coatings that were deposited with no maskemployed (curves a and b), their maximum thickness occurs at the centerof the spherical surface. To a very close approximation, their thicknessdecreases along a parabolic curve out to their perimeter. This decreasein their thickness may be corrected by employing the masks 20 discussedearlier, based on FIGS. 1 and 2. As has been explained, that will allowdepositing coatings that exhibit, for example, either an essentiallyuniform thickness distribution or a moderate, parabolic, increase inthickness from their center out to their perimeter (cf. FIG. 5), whichdemands a rather smooth thickness distribution, which, in turn, demandsthat the shielding masks employed in order to preclude large incidenceangles must be chosen such that they cause no kinks or jumpdiscontinuities in their thickness distribution. As may be seen fromFIG. 8, the annular mask 127 causes no prominent kinks in theirthickness distributions. Although their thickness distribution isparticularly smooth in the case of an approximately centered evaporatorposition (curve c), the demands on mask positioning accuracy become morestringent. The optimal eccentricity offset 131 may be empiricallyestablished. These computations show that the aforementioned values ofaround 50 mm are beneficial in the case of systems of the indicatedsize.

The combination of a centered shielding mask having circular rims and anoff-axis material source presented here in the form of an examplerepresents merely one possibility for obtaining the described benefits.Yet another means for limiting mean particle incidence angles would beproviding a slightly off-axis, annular, mask and centering the materialsource. The important thing is the slight decentering of the circularrim of the mask with respect to the material source. Combining acentered shielding mask having a slightly elliptical rim with a centeredmaterial source is also feasible. Yet another possibility involvesslightly decentering both the material source and the rim of the maskwith respect to the primary rotation axis, subject to the condition thatthe material source and the rim of the mask are slightly decentered withrespect to one another over at least one or more sections of the rim ofthe mask.

Yet another opportunity for reducing the maximum mean incidence angle incases where a planetary-drive system is employed involves providing adevice for tilting the rotation axes of the substrate carriers, asindicated, for example, by the double-headed arrows 135 appearing inFIG. 6. For coating the convex surfaces depicted there, the planetsshould be tilted outward; for coating concave surfaces (cf. FIG. 10),they should,be tilted inward. This tilting causes the maximum meanparticle incidence angle to occur at the center and perimeter ofspherical, or aspherical, curved surfaces. However, if this tilting ofthe planets is optimized such that these two maxima, i.e., that whichoccurs at the center and that which occurs at the perimeter of suchcurved surfaces, are approximately equal, then the range of meanincidence angles that occurs over such curved surfaces may be minimized,which will also minimize variations in the optical constants of coatingsdeposited on such curved surfaces over those surfaces.

FIG. 9 presents plots of the mean incidence angles β of particles on aconvex, spherical, surface having a radius, r, of r=150 mm for the caseof coating using a planetary-drive system of the type presented here forvarious configurations. Compared are coating it without using a mask andwithout tilting the substrate carriers (curve a) and coating it using anannular shielding mask, both with no tilting of the substrate carriers(curve b) and for the case where the substrate carriers are tiltedoutward through an angle of around 20° (curve c). Although it is clearthat variations in mean particle incidence angles may be greatly reducedby tilting the planet axes (substrate-carrier axes), local mean particleincidence angles increase over large areas of the spherical surfaceinvolved.

From FIG. 10, it may be seen that the invention may also be used forcoating concave, spherical or aspherical, surfaces 211. In the case ofsuch surfaces, those areas where the largest particle incidence angles βoccur lie near their perimeter and on the side facing the primaryrotation axis 203. The embodiment schematically represented in FIG. 9employs a shielding mask 227 that is essentially configured in the formof a circular disk or circular plate having a central, circular, rim 230centered on the primary rotation axis 203 for reducing mean particleincidence angles. Both the height of this shielding mask 227 between theoff-axis (by an eccentricity offset 231) material source and thesubstrates and the position of the diameter of the disk defining the rimof the mask should be chosen in much the same manner as in the case ofthe geometry shown in FIG. 6.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. It is sought, therefore, to cover allchanges and modifications as fall within the spirit and scope of theinvention, as defined by the appended claims, and equivalents thereof.

1. Optical component comprising: a substrate having at least one curvedcoating surface to which an optical coating that is rotationallysymmetric with respect to a symmetry axis is applied, wherein theabsolute value of a ratio of a diameter of the coating surface to aradius of curvature of the coating surface exceeds approximately 2/3,wherein a density of the optical coating varies by less than 20% over aradius of the coating, and wherein the optical coating has a physicalthickness continually increasing in a radial direction, from thesymmetry axis to a perimeter of the optical component.
 2. Opticalcomponent according to claim 1, wherein the density of the opticalcoating differs from a mean density of the optical coating at a locationof the symmetry axis by less than ±10% over the radius of the coating.3. Optical component according to claim 1, wherein the optical coatinghas a physical thickness monotonically increasing in a radial direction,from the symmetry axis to the perimeter of the optical component. 4.Optical component according to claim 1, wherein a physical thickness ofthe optical coating at the perimeter exceeds that in the vicinity of thesymmetry axis by at least 5%.
 5. Optical component according to claim 1,the optical component being one of a lens and a mirror.